1. Field of Invention
This invention relates generally to flexible polyurethane foam material of the open-cell type, and more particularly to a low-density flexible polyurethane foam body which is modified to impart resilience and enhanced shock absorbing properties thereto.
2. Status of Prior Art
It is known to use synthetic plastic flexible foam material as an underlay for carpets and rugs, for automotive seating and padding and in many other applications requiring cushioning and shock absorbing properties. The materials usually used for this purpose are polyurethane foam, foam rubber and polyvinyl chloride plastic material.
Among the advantages of polyurethane foam over vinyl and other commercially-available forms of foam plastics, are that polyurethane foam has markedly superior thermal and acoustical insulating properties as well as a more uniform cell structure. Moreover, not only is polyurethane foam much lighter than vinyl foam, a significant factor in handling and transportation costs, but it is a far more economical material.
Many practical applications exist for a polyurethane foam product of low-density having some degree of firmness and good load bearing characteristics. A product having such properties would be useful, for example, in automobile seat cushions, upholstery and bedding. This combination of characteristics cannot be obtained when the foam is derived from a standard polyol intermediate; for to achieve firmness, the foam must then be of a higher density. The resultant weight of the higher density product is objectionable in many applications, to say nothing of costs which go up substantially with an increased density.
One can produce relatively low cost, low-density polyurethane foam using a polymer/polyol intermediate in the formulation to obtain a very firm product having excellent load bearing characteristics. Polymer/polyol intermediates suitable for this purpose are disclosed in the Seefried, Jr. et al U.S. Pat. No. 4,111,865. These compositions are made by the in situ polymerization of a vinyl polymeric base, to give a dispersion of the vinyl polymeric portion in the liquid polymer. Polymer/polyols are characterized by the presence of polymer-to-polyol grafted species. Polyurethane foam products are made from the polymer/polyol grafted species.
Polyurethane foam products made from the polymer/polyol compositions disclosed in the Seefried, Jr. et al. patent are less susceptible to static fatigue, so that when the load imposed thereon is lifted, the foam returns to its original unloaded state and does not remain deformed.
The degree of firmness of a flexible polyurethane foam product is defined by its indentation load deflection properties (ILD). Thus when the ILD is in the range of 18 to 24, it is classified as soft; when the ILD is in the 24 to 30 range, it is medium soft; whereas the 30 to 36 range affords medium firm properties; the 36 to 46 range, firm properties, the ILD's above 46 being very firm. Foams made with polymer/polyol intermediates fall into the firm and very firm ranges and are not suitable for those applications which require less firm and medium soft ILD's of low density foam.
It is known to modify polyurethane foam to alter its dielectric characteristics so as to impart thereto a loss factor which lends itself to dielectric heating techniques. One approach is that set forth in the Schickendanz U.S. Pat. No. 3,061,460 which involves the post impregnation of urethane foam of the ester or ether type with a vinyl plastic to so alter the dielectric properties of the foam as to render it dielectrically heatable.
Another approach is that disclosed in the Weisman U.S. Pat. No. 3,244,571 in which the polyurethane foam is modified by the introduction of vinyl resin. This is accomplished by including vinyl particles in the foam-forming reaction mixture. In this way, the vinyl is diffused during the foaming process throughout the fibrous structure of the foam without filling the cells thereof, so that the structure of the foam retains its normal cushioning and acoustic insulating properties that would otherwise be degraded had the cells been impregnated. Other examples of post-impregnation may be found in the Dugan U.S. Pat. No. 3,393,119, the Fishbein U.S. Pat. No. 3,535,197 and the Hand U.S. Pat. No. 3,585,062. Also of prior art interest are the Weisman U.S. Pat. Nos. 4,482,582 and 4,525,386 in which polyvinyl particles are impregnated by forced air into the open cells of polyurethane foam to impart dielectric heat-sealing properties to the foam. But such modification of the polyurethane foam does not impart resilience thereto.
A conventional, low-density polyurethane is lacking in resilience, hence it is not fully suitable as a carpet underlay or in similar cushioning applications where when the carpet is stepped on, the foam tends to bottom out, and does not recover its initial form.